Your Ultra-Training Bag of Tricks: Unbonk!

Recently Dana Pearcy participated in a local 35K trail race as a training run. She finished the event but had to gut out a bad patch that lasted for several miles. “Around mile 14 I started feeling really dizzy and the trail became blurry and surreal looking,” she recalls. “My mind was having trouble processing the trail as I was trying to move across it. I was forced to slow down so I wouldn’t fall. This continued for about 4.5 miles. Around mile 18, I fell, not too bad, just minor scrapes, because I felt super clumsy trying to move through the riverbed I was in. I stopped at the next aid station for a few minutes, ate, and hoped my mind would clear. Shortly after that the feeling passed, but for the rest of the race I felt slow, out of it, and had a hard time focusing.”

Many of us can relate to her story. We’ve all “bonked” or hit the wall. It can creep up on us no matter our running speed, effort, or training background.

Biology 101

In order to appreciate how the bonk occurs, an abbreviated biology lesson is in order. The body uses carbohydrate, fats, and proteins to create muscle movement. Enzymes convert these nutrients into ATP (adenosine triphosphate). ATP doesn’t stick around in the body. It’s created and used in a matter of seconds. Energy, which allows muscles to function, is released through the continual formation and subsequent break down of ATP.

Carbohydrate, stored in the form of glycogen in our muscle, liver, and blood, is the primary, quickest, and most accessible fuel source for medium to high-intensity activities. The body can turn glycogen into ATP without using oxygen. The unfortunate by-product of anaerobic metabolism is lactic acid, that burning feeling in the muscles when you’re running at high efforts. The trained human body can store enough glycogen for about two hours of hard running.

If we slow our pace, allowing more oxygen to become available to the muscles, we can run for greater distances. Fat, instead of glycogen, is metabolized during these endurance sessions. Fat yields the biggest ATP return and is our most abundant fuel supply, enough to keep us going for days. However, small amounts of glycogen are necessary to complete the fat utilization process. Protein, used for muscle repair, is seldom used as an energy source.

So, we’ve got this remarkable system that burns different fuel sources depending on our effort level and exercise duration. The trick is managing our limited carbohydrate stockpile by using our bountiful fat reserves. If we don’t account for glycogen loss in our training and racing programs, we will hit the wall. “Technically, it’s the depletion of your stored carbohydrate energy or glycogen,” says Robert Kunz, who holds a master’s degree in exercise science and sports nutrition and is the co-founder of First Endurance. “Once you run out of glycogen your body must rely on fat and, to a lesser extent, protein. These fuel sources are hard to convert to energy and hence the athlete’s pace is reduced significantly. Some feel queasy and dizzy. It can be hard to think clearly due to the lack of readily available blood sugar.”

Stop the Bonk

There are two reasons why we deplete our glycogen stores while racing and training. Assuming we’re healthy and not fighting infection or injury, we struggle in the late miles of an event because we’ve 1) exceeded our current fitness level by running at too high an effort, or 2) neglected to replace adequately our precious carbohydrate sources. Here are three ways to prevent both scenarios:


Our tendency on race day is to run too quickly too early. Keeping our effort more in tune with our current fitness will extend our ability to run a steady pace for longer. “Athletes use primarily fat at slower paces. As the pace increases so does the reliance of stored glycogen. That is why pacing is so critical,” says Kunz. Three telltale indicators that you’re tapping your glycogen stores during exercise are forceful, quick breaths, the inability to carry on a conversation, and the accumulation of lactic acid in the muscles. Quite simply, you’ve become anaerobic; your muscles are no longer using oxygen to create energy. Slow down, return oxygen to the muscles, and allow your fat stores to take up the slack.


Bonk proof your body with years of consistent aerobic exercise. Kunz, who’s finished the Speedgoat 50k, agrees, “The longer you prepare with proper distance training and racing the better your body adapts to the distances and efficient glycogen utilization. This is why athletes should do a 50k before they do a 50-mile and use a similar progression to get to 100 miles. It takes years of training to fully adapt to these distances. An elite ultramarathoner can run a marathon on just a couple hundred calories. Yet if a healthy 20-something jumped in, he would bonk completely by mile 20. The college student does not have the years of adaptation and hence his body burns glycogen at a much earlier stage than that experienced ultrarunner.”

Start with endurance-based workouts or long runs to:

  • Train the body to become efficient at burning fat, its optimal fuel source.
  • Improve your ability to amass muscle glycogen, the major form of stored carbohydrates in the body.
  • Increase the size and number of muscle capillaries and mitochondria, the blood vessels and cellular factories that facilitate aerobic energy.
  • Learn to keep going when fatigued.

Challenge yourself with carbohydrate-depleting long runs to teach your body to use fuel stores more sparingly and perform economically with low blood sugar. “The theory is that through many months of training on few carbohydrates you adjust your ability to burn fat more efficiently,” says Kunz. “The more fat you burn, the less glycogen you burn.”

Introduce stamina-based workouts into your schedule to further your body’s ability to utilize both oxygen and fuel well. Shift your lactate threshold toward faster speeds and harder efforts.

 Fueling and Hydration

Stockpiling your body’s fuel sources begins well before you toe the start line. Reduce your training volume and maintain a regular meal schedule to allow for muscle recovery and glycogen storage. See this article on proper peaking for more details on how to approach the final weeks before an event.

On race day your goal should be to prevent glycogen stores from being used rather than trying to replace them by consuming fast-absorbing carbohydrates such as gels or your favorite sports drink containing maltodextrin or similar. As easy as it sounds, keeping the bonk at bay is a tough challenge. Here are some common indicators that you’ve hit the wall:

  • leaden legs
  • acquiring a grumpy, down-trodden attitude
  • dizziness
  • tingling lips or cheeks
  • clammy skin
  • lack of motivation
  • lack of power
  • temporary loss of or blurry vision, seeing “stars” or white flashes after blinking

If you do find yourself in this predicament, Kunz recommends, “Immediately consume fast-absorbing sugars and water. Remain at a walk so you allow your body the energy it needs to process this and drive it to the working muscles. After roughly 15 minutes, slowly pick up the pace. At this point you might be hard pressed to bring yourself back up to your normal race pace since you do not have any stored glycogen to pull from. From this point on your body will rely on fat and the oral carbohydrates you’re taking for fueling purposes. You still have plenty of fat energy left and can keep going for long stretches, however it’ll be at a much slower pace.”

If things begin to feel dire and hopeless during your next ultra remember this: The brain is the only organ in the body that operates solely on blood sugar. A severe case of glycogen depletion can starve the brain leading to a loss of concentration, confusion, clumsiness, a bad attitude, anger, and depression. Before pulling the plug sit down, drink, eat, and wait 20 minutes. Sometimes a little food and drink can revive you enough to get you to the finish line.

During the 1998 Western States 100 Mile Endurance Run, I bonked so badly that I couldn’t even walk. I spent 45 minutes at the Peachstone Aid Station gathering myself. After some food and fluid (the flute music and Reiki were also nice bonuses), I was able to get off the cot and continue, albeit at a much slower pace and a better eye on my fueling.

While driving, we diligently monitor the gas gauge. Do the same for yourself at your next ultra. Keep close watch over your body’s energy levels, so that gauge never reaches empty.


Moxley, Cathy. Runners Beware: Are You Wasting Precious Carbs When You Could Be Burning Fat? Washington Running Report, Mar. 2008.

Quinn, Elizabeth. Energy Pathways for Exercise – How Carbohydrate, Fat and Protein Fuels Exercise. Sports Medicine, Jan. 25, 2008.

Torrence, Ian. Your Ultra-Training Bag of Tricks: Endurance-Based Workouts., May 1, 2012.

Torrence, Ian. Your Ultra-Training Bag of Tricks: Stamina-Based Workouts., 5June 5,2012.

Torrence, Ian. Your Ultra-Training Bag of Tricks: The Difficult Art of Peaking., Sept. 4, 2012.

Call for Comments (from Meghan)

  • Okay, fine, Ian’s article calls for it. Bonking horror stories, who’s got one that tops Ian doing Reiki at Peachstone as he awaited his own unbonking?
  • What have you found works best for you in terms of staving off a bonk in the first place?
  • And where in your pacing, training, and fueling/hydration could you do better to avoid the dreaded energy bonk?

There are 60 comments

  1. Charlie M.

    Another key is to age. Now that I'm in my 40s I don't bonk. Not just because I've learned through trial and error (although that is definitely a piece of the puzzle), but because my body seems more geared toward slower paces, more efficient fat burning during endurance events, and more resistant to bonking in general. I watch my 5-year-old son run around and all he does is bonk–he runs out of energy after intense bursts. His ATP system must be different than mine! And why are there not many teen-age marathoners/ultra-marathoners? Because they are designed for speed, not endurance.

    So the real key is to get old. Bonking goes away (in more ways than one, wink wink).

  2. Joey Schrichte

    What are some examples of "fast-absorbing sugars" that Kunz refers to? I know Coke is one, but what are other examples in case Coke is not available at an aid station?

      1. Ian Torrence

        I add to Anon's comments here by saying that not all gels and sports drinks are created equally. Maltodextrin, fructose, and sucrose are the sugars you're looking for. We could perhaps do without the added colors, flavors, and gelling agents.

          1. Ian Torrence

            Tony – You can purchase maltodextrin as is (a white powder) and mix it with water. Search for it on Amazon or try Carbo Pro. You can order small bags of it or an 8 lb vat. They'll love you at the airport security line. I'll also add that it wasn't a coincidence that I interviewed Robert Kunz from First Endurance for this piece. I really like what they've got going on over there. I use malto + their EFS Drink mix for a nice long run drink. FE also just posted an excellent article on fueling for 100 milers: Check it out!

            All the best! Ian

            1. Guy Smiley

              Something I recently discovered are dates. All the other usual stuff works, but dates have an added component (anecdotally, for me at least) – they settle my stomach. They have a weird texture but a very pleasant aftertaste, they melt in the mouth and they provide instantaneous energy.

              And, my stomach feels happy after. That's always good.

  3. Shelby

    As a marathoner and newbie ultrarunner, I've never bonked. I assume this is due to eating early & often, using handhelds which remind me to drink regularly and running at a pace I know I can sustain, which comes with experience (I hate HR monitors!). Maybe getting old helps too, as Charlie said. ;-)

  4. SkyDog

    Looking back on my recent first 50 miler, I ran on about 2700 calories total. Bonked out of the last aid station but made it in. Felt an incredible desire for food and was trying to make it to a restaurant but got floored with sudden naseua and passed out in my car. Lesson learned…

  5. Anonymous

    Ian, I am eschewing your advice/comment on the natural progression to the longer (in my case, 100-mile distance), having only ran two 50K's. Do you think I am 'doomed' for failure? I am going to celebrate my 50th birthday by running this event, so I have years of miles in my legs. Would appreciate any comments you may have for me if you scan these posts.

    1. Ian Torrence

      Anon – Nah, you're not doomed to fail. Set reasonable goals, pace yourself, eat and hydrate well, and listen to your internal "dashboard". Use those years of experience to your advantage. Comment back here after you've finished and share your experience! Have fun and good luck! Cheers! Ian

  6. Andy

    Great article, Ian, as always. Intelligent, informative, but not over the heads of those of us without an advance degree in exercise physiology. The advice for bonk prophylaxis and intervention is pretty conventional, but at least now I understand the bio basis a bit better.

    As for bonk stories, I am a 49-year-old runner who has been a Type I diabetic since age 3. I can't tell you how many ultras (along with a few marathons and an infinite number of training runs) I've run (and finished) with blood sugars all over the map, as low as the 40s after a few finishes and 60s at various aid station stops (yeah, glucometer in drop bags is a good thing). It's true that pushing your body to run with low blood sugars (in my case, not intentional!) definitely helps the bodies energy utilization system to adapt.

    1. Ian Torrence

      Andy- I actually thought about touching on your situation (diabetics) in these piece, but thought the better of it. My knowledge and experience only goes so far, I didn't want to complicate things, or send out incorrect information. Thanks for sharing! Ian

  7. Steve Pero

    In this sport, particularly, it is wise to heed your first bit of advice:

    "Train the body to become efficient at burning fat, its optimal fuel source"

    Most of my training now is at pretty slow paces, between 10 mpm and up, yet in races I can usually finish in the top 30% and I'm 61. In racing I rarely consume much at all but water until I need it, but always have several gels with me as an emergency bonk fixer. Back in my younger days of sub 2:50 marathons, I bonked plenty, but you have to go there when racing fast.

    One example of a good training run I did last year (which is similar to most of my long runs)…Deb and I had some friends up from Texas who wanted a tour of the Sangre DeCristo mountains near Santa Fe. It was a 20+ mile, 5 hour typical mountain workout and our friends were easily 20-30 years younger than I. On all of the climbs I fell behind their pace, but was able to catch them on the downhills. We were up above 12K for a good part of the day and they were sucking down gels every 15 minutes, while I ate nothing. They wondered about that. When we got back to the parking lot, they asked why I didn't need to eat anything and I told them I burn fat as fuel…while I drank my Coke and ate some potato chips, my favorite post run fueling ;-) Just saying that to race fast, you need to slow down in your training…it works!

    Thanks for another great training article, Ian.

    1. Anonymous

      (I'm looking forward to running with you in the GC in April, and thank you for the invite last week, about to turn 50 and have a few questions about things you seem to be knowledgeable about — Art does most of our thinking and planning, but I'm the one with the crazy ideas!)

      JV in SD

  8. Hone

    I once ran a 50 miler and only took in 3 gels the entire race. That was the worst experience of my life.

    This article just opened my eyes about why I stink at ultras.

  9. Patrick McKenna

    What do you think about the Maffetone training methods Ian? (180-age, etc.) to establish training zones.

    Also, what about people who say they naturally have a higher heart rate than the calculation estimates?

    I know that is a little off topic but it does relate to fat burning, staying aerobic, pacing, etc.

    Great article BTW.

    1. Ian Torrence

      Patrick – The Maffetone Method ( is an excellent training approach for runners who may have a hard time gauging their efforts and/or are new to the sport. It forces you to work within your limits and to progress slowly and safely as you develop. For more experienced runners I believe the watch and perceived effort are better tools to use. I do, however, often suggest that athletes (no matter their ability level) who run their easy and recovery days at too hard an effort use a HRM so they stay in check. If you are truly wishing to learn more about Maffetone and running I'd reach out to Tim "Lucho" Waggoner ( as he seems to know it inside and out. I hope this helps. Ian

  10. J.Xander

    Timely article:

    I prefer to run but in the winter try to take a break and skate ski much more often. For the third time ever I participated in a 50k ski race this weekend. (50k ski race is about the time equivalent to a marathon if you are decently experienced -which I am not). Anyway for the third time ever I BONKED! Hard! I have never bonked during a run, ultra or otherwise, but I have now bonked in all my ski races. This has left me totally baffled.

    I bonk, literally 2 miles from finish, right before the last big climb. This time I saw stars, wanted to lay down in the snow bank, take off the skis and just generally fall asleep. I immediately ate a gel and pushed myself, cramping and slogging (about twice the pace I had been keeping) to the finish, where I collapse, crawl to the car and eat everything I can.

    My theory is that I don't take the time to eat regularly during a ski because the damn ski poles make it difficult without actually slowing to a stop to take the poles off. In a run my hands are free so eating doesn't require me to slow down. I am not sure if this is it and it still baffles me. I am fit enough to run the distance but for some reason skiing the distance I always meet the wall.

    1. Andy

      Perhaps also that ski races are in colder climes — hence, less thirst, less awareness of fluid loss, less fluid and calorie intake overall?

    2. JKuz

      Fueling is more difficult during nordic races, not only for the reason you mention (poles make it hard to use your hands), but the cold temps freeze water bottles and harden gels to a hard chew consistency sometimes. Also the quadripedal nature of nordic ski racing may consume glycogen at a higher rate due to higher core and upper extremity muscle use than in running. Afterall, nordic skiing has the highest aerobic component of all endurance sports due to the largest amount of muscle activiation. A lot of experience is needed to learn pacing since there are areas where you are gliding and uphills where you are really hammering. Overpacing the race will cause a bonk no matter what.

      With practice, fueling can be improved. Use gels that are more liquid ( ie.Powerade is easier to suck down than Hammer or GU when the temps are below 20) and take advantage of the aid stations where they will sometimes warm the liquids. It takes practice to grab the cups with poles but use both hands and you will get more in.

      If you carry a fuel belt that doesn't manage to freeze, take advantage on those downhill glides to take a swig.

      1. Andy

        I'm not a regular nordic skiier but do run all winter (of course) and snowshoe when the stuff's too deep. I always keep one gel inside the palm of my glove to keep warm. Just a little trick that works for me. The poles, however, are another matter …

  11. Michele Sun

    I just finished Tokyo Marathon and I felt that dizziness after 35k and got muscle cramps oy quads. Now reading this article I can see where I have sinned. Dehydration!!! I was trying to make up my time by skipping aid-stations!! Mistake!

  12. PoDog

    This is the first time I've read that your body gets used to having fewer calories. I've been experimenting for the past couple of years with fewer calories, and I need a lot less colories now that I did several years ago. I just did a marathon on no gels and about 300 total calories and a 50k on 1 gel and about 600 total calories. I need fewer calories during my 100 milers too and I cannot remember the last time I bonked.

  13. Pete

    Very interesting article to me as I am in the process of trying to move from being a sugar burner to a fat burner in daily life. The process is not easy but I am hoping to get better results and in theory this should teach my body to use only fats while running. I will be doing my first real long run on saturday with just water as i go through this transition. I am very nervous to see if i can run 25 miles on just water while i could bonk i think it will spring board me into a strong running season with fewer stomach issues and the ability to need fewer calories.

  14. Matt Hart

    Great overview Ian. This should be helpful to many.

    Two things I noticed were incorrect on the science side:

    1) Lactic acid is not a caustic byproduct, it’s actually a fuel. Our muscles make it deliberately from glucose.

    2) The brain can run just fine on ketones. Although I wouldn’t suggest anyone try to do that in during a race.

    1. Sage Canaday

      I think only the lactate can be used as a fuel….but when the H+ ions attach themselves (it becomes lactic acid) and the drop in blood PH makes it harder for the muscles to function…

      Of course that accumulation isn't the limiting factor in any race over about 20-22 miles…fuel ultilization will be…

      Ian, don't you start burning fat as a fuel after about 25-30min of running? So during a 100km I'm burning both fat and glycogen basically the whole time (at different %'s of course) but refueling with sugar along the way as the glycogen goes from the liver. Basically it becomes an eating contest with lots of running…

      My worst bonk story: The ING NYC marathon…" I went for it" [also took no gels] and doubled my min/mile pace for a couple miles after mile 21. I was so dizzy my main concern became not falling flat on my face and getting another concussion!

      1. Ian Torrence

        Matt and Sage,

        You guys are both right. Anaerobic (without oxygen) exercise produces, from glucose, lactic acid and the H+ ion. Matt's correct, lactic acid is a fuel that can be utilized during a tough bout of work for roughly 30 seconds to 3 minutes…in training terms that'd be a classic example of a VO2max interval. And Sage is correct, the H+ ion (acidic) released in that non-oxygenated environment creates that burning feeling in the legs (lowering blood pH). We need to rest after that short interval so our body can clear the acid and we can begin again. Shame on me for making the biology lesson in this piece too abbreviated.

        Sage – I believe a trained athlete can begin burning fat even faster than that. I was tested once when I lived in Seattle, and they found that I began the fat burning process four minutes into a run! You bet, glycogen is needed to burn fat. I think I mention that in the piece.

        Matt – I didn't know about ketone's function with the brain. Fascinating stuff! However, as you alluded to, if your brain is running just fine on ketones, chances are your body isn't fine.

        Thanks for the responses. Psyched you guys are reading this stuff!


        1. KenZ

          I've listened a VERY interesting podcast interviewing a guy who put himself into a permanent state of ketosis for like a year or more just to see how it works. He posted all the info on the web, including blood work, etc. A basically zero carb diet… and it works. The podcast I've listened to, he does NOT claim that it is healthy, or optimal, but was doing it more from a scientist N=1 standpoint. His main outputs of interest were:

          A. He did not die, and seemed REASONABLY healthy, although I can't imagine it's good for you. (and again, he wasn't saying it was good for him)

          B. He has zero explosive power. None. He said it would be a terrible way to be in a race like cycling where you could draft because you'd never be able to do a short sprint to catch a pack or anything like that.

          C. He could go forever on pretty much zero food; there was no bonk, because he'd bonked like a year ago.

          D. He'd go out for a 6 – 10 hour bike ride with friends. They'd be wolfing down gels, bananas, whatever, and he'd do maybe one beef patty, or a quarter stick of butter, or… NOTHING for the entire ride and be fine the whole time.

          I think the best takeaway for me was that it shows, once again, how adaptable the human body is. We all talk about how different things work for different people (vegan, paleo, high carb, low carb, whatever). My postulation is that perhaps many, if not all things, could 'work' for most people because humans are so damn adaptable. One just needs to find what it is that they WANT to do, and what fits in with their lifestyle.

          That stated, you are totally right that for those of us not on a no-carb/ketosis diet, there are some physiological fundamentals that one cannot escape if you want to avoid the bonk.

  15. Sarah

    Excellent advice Ian, thank you! I was in the same boat as Hone and thought 4 gels would get me through a 50k, well I made it but it wasn't very pretty. I'll definetly pay more attention to my fuelling this year!

  16. Lizzy

    I'm a runner second, a climber first, and most of my worst bonks have been on long alpine climbing days. For some reason, when I climb I never get hungry and there are so many other tasks to do that I often forget to eat. It doesn't help that when I start bonking I get nauseous and grumpy… when I am offered food I usually respond with "I hate [food that I normally love]" and get angry at my poor partner for trying to help me. When I run, I am much better at noticing a bad attitude starting, feeding myself, and not fully bonking.

    As a runner, my worst bonks have been on high altitude runs… I feel like my stomach is more of a PITA above about 7000ft (that's high for us Californians). Is there anything I can do about it other than experimenting with fuels that work better for me under those conditions?

  17. Ian Torrence


    Thanks! Do you have the links to this podcast and website? It'd be interesting to see his transformation. I did some searching as well when Matt mentioned it. I found that there might be a connection that long states of ketosis could help epilepsy or Alzheimer's patients. Many of the studies failed because the subjects couldn't deal with the diet required, like glasses of cream and sticks of butter.

    I agree that the human body is an incredible adaptor. However, let's not forget that not all adaptations are beneficial. A long time diet of butter, cream, and beef patties probably isn't the best thing in a "long run."


    1. KenZ


      Here's a link to the podcast, and it's REALLY good. The guy who was doing the self experimenting is an MD (Stanford), and REALLY geeks out on the science of it. Like, taking blood glucose tests every few hours to check level, looking at how sodium levels are required to achieve good results, how too much protein (!!!) can sabotage a ketogenic diet, etc. He states that ~90% of his calories over a day come from fat. You read that right: 90%!!!! The guy is no quack; he's a scientist experimenting on himself and providing very interesting results.

      You'll never see me try it, but again, it shows how amazingly adaptive the body can be. Do write back here as to your thoughts after you listen to it.

    2. KenZ

      PS- he also has a nice clip talking about bonking, and why in a state of ketosis, he doesn't/can't bonk, because he always has access to fueling from his fat stores. He is truly bonk-proof.

  18. UltraDad

    Great Advice, Ian, Thanks.

    A current issue that I'm faced with is that I get slowing down in training to become a better fat burner, but that seems to play out as a slower race time too. How can I get better at burning fat AND try to improve my times?

  19. UltraDad

    I feel that I have gotten good at slowing down my long training runs, but feel that I've also gotten good at racing slow as a result – what's your secret to training slow and racing fast?

    1. Steve Pero

      Knowing when to add in the faster workouts. I generally will spend several months at the turn of the year (after the prior race season) repairing and rebuilding my aerobic base by running with a strict Maffetone protocol. This can be humbling, but it works…after a few months of this low HR training, I can move into doing hill sprints and longer hill repeats, then move into tempo runs. Usually mid year I have to return to the easy stuff for 4-6 weeks for more recovery.

      I ran sub 2:50 marathons and sub 35 10K races when I was younger, just running easy to and from work, adding in occasional races, but no speedwork.

      Check out Maffetone's Big Book of Endurance training and racing.

  20. Eric Coppock

    Wow, the "Biology 101" portion of this article is pretty far off the mark. Two major misconceptions are built in to this discussion:

    1) The body switches between "aerobic" and "anaerobic" metabolism at some threshold intensity level. Not true. Both mechanisms are active at all times, the *blend* is what varies with intensity level. When you "go anaerobic" this really just means that the fuel-burn blend is now far enough toward the anaerobic that you're producing by-products faster than you can clear them. You've crossed a *sustainability* threshold.

    2) "Aerobic" metabolism burns primarily fat, "anaerobic" burns primarily carbohydrate. Well, the second part of this is true. Carbohydrate is burned by both aerobic and anaerobic processes. Fat and protein each have their own processes (both involve oxygen). Again … there are multiple fuel mechanisms in play, and the blend between them changes with intensity of effort and time.

    The "what to do about it" parts of the article seem reasonable enough.

  21. Evan

    Well said, Ian! I recently bonked in a race- the American Canyon 50k- despite being well-trained on low calorie and carb intake, because I went out much too hard for my current training at the beginning of the race. Always a work in progress.

  22. StephenJ

    I'm a backcountry telemark skier in the winter time, because the Wasatch mountains are my backyard. I drink EFS sports drink from a hydration pack for my calories. The EFS drink also seems to have a much lower freezing point than plain water. Hydration tube on the way up, avalung tube on the way down.

  23. Rob Sargeant

    I ran a 56k last September on a warm day, taking in nine gels over the course. The last one I took with roughly 10K to go. I was in 8th place at that point. I kept drinking Gatorade at the aid stations heading toward the finish, but was loosing my kick and my legs felt like lead. In the last kilometer I was passed by three runners. I think if I had had 2-3 more gels and some S-Caps I could've put up a better fight.

  24. Emily Cooper, M.D.

    I'd just like to clarify that the concept mentioned in the article and some of the comments that it is helpful to run with low blood sugar as a strategy to 'train your body to adapt' is a very flawed concept. This is NOT a healthy or wise strategy under any circumstance for athletes and leads to acute brain injury, low testosterone in males, low estrogen in females, overtraining syndrome, slow metabolism, increased body fat and lean mass loss and poor performance. There is an abundance of misinformation that athletes are exposed to and as a physician who has helped hundreds of athletes recover from overtraining syndrome and other metabolic mishaps I felt the need to jump in with this clarification.

    1. Andy Meisler, Ph.D.

      Dr. Cooper — I think your comment is important and underscores the risks and consequences of "bonk training." Still, with all due respect, is it not possible that it might still actually increase one's ability to "run on fumes", negative consequences notwithstanding? Just trying to look at the medical science and separate benefits (if any) from costs. Your cautions would certainly urge folks not to use this as a regular strategy in training.

      On the personal side, as a Type I diabetic I have run (and lived) with very low blood sugars intermittently all my life, so far with no impairments that have manifested clinically. But then N of 1 studies are always limited …

    2. Ian Torrence

      Hi Dr. Cooper –

      Thanks for your input. By no means do I wish to insight an epidemic with this article. However, I do believe, as I suggest in my article above, that carbohydrate-depleting runs, when done intelligently (follow the link provided above) can be a valuable exercise for a runner. Wouldn't you agree that there is probably more at play when we start seeing "…acute brain injury, low testosterone in males, low estrogen in females, overtraining syndrome, slow metabolism, increased body fat and lean mass loss and poor performance" in runners? Might these stem more from some of the other troublesome issues runners face like body image and eating disorders? I think a runner in good health who takes care of themselves both "on and off the field" can benefit from learning what it feels like at the end of an ultra-distanced event. Carbohydrate depleting runs do that in a safe and controlled environment.

      Thank you for your important response.


      1. Emily Cooper, M.D.

        Hi Ian,

        I will make one more comment to hopefully provide additional perspective and I realize this is a touchy topic because people are really invested in their approaches. Fads involving dietary advice come and go and when they come they can sweep up a lot of people. However, I feel obligated to try my best to warn readers in this area in an effort to avert potential disasters like those I see regularly in my practice day to day, week to week. I do have credibility in this area as a physician who is board certified in family med, sports med and obesity medicine, having seen thousands of athletes over the past 20 years, performed over 3500 metabolic tests on athletes, conducted numerous research studies for fitness equipment companies, a team physician and running medical tents at ultra-events.

        I think your article is not inciting an epidemic, the epidemic is already in full force if my practice is an indicator. It's simply not supported in the large body of science that it's healthy or advantageous from a performance standpoint to minimize carb intake in an effort to 'train your body to burn more fat'. Of course if you starve your body of carbs, they won't be available for fast efforts which require carbs not merely fat so you will be locked in to fat burning which can support endurance but has limitations with regard to speed. Optimal fat burning is great but going for very high fat burning does not necessarily translate into good or better performance. While restricting carbs and depleting carb stores may not impair endurance as long as speed is slow enough for fat to serve as fuel, speed and performance will be impaired and health will be impaired. In our lab we call this physiologic state 'excessive fat burning' or 'deficient carb burning' on metabolic testing and this is linked to 'inefficiency' or greater O2 requirement for slower speed and development of a 'false ceiling' in terms of performance capacity due to a significant depletion of the anaerobic capacity which contributes to speed performance at workloads from the low level of aerobic threshold (multi-hour performance) to the max level of anaerobic capacity (40 sec sprint). Training this way produces a 'one-gear' type physiology with a collapse of the spectrum of gears from the AT to max. The body senses this collapse and cannot perform at maximal steady state with as much consistent power output as it would if those gears were present.

        There is a trend among ultramarathoners to have a compressed upper range which limits their improvement capacity. Much of this is due to either lack of speed training with adequate glycogen or more commonly training in the presence of inadequate glycogen stores. I've tested VO2max and physiologic performance in several hundred ultraendurance athletes and have done over 3500 physiologic tests in my career and have a wide knowledge of physiology of sport performance and athletic medicine. There is no good reason for an athlete to purposely train in a low carb state and the overwhelming body of scientific evidence supports this viewpoint.

        Also, people don’t realize that the definition of disordered eating is a mis-match between expenditure and intake and there is a fine line between disordered eating and eating disorders among athletes in terms of the physiologic impact. If athletes are wondering if they are fueling adequately, have labs checked – AM fasting WBC, absolute neutrophils, leptin, Complement C3, Total T3, LH, cortisol, testosterone free and total for males and LH, FSH, Estradiol for females. These results need to be assessed by a physician who is versed in athletic medicine and sports physiology.

        The effects on metabolism outside of the workout are also detrimental, especially for female athletes. While males who train with inadequate carbs may develop low testosterone, females can set themselves up for extreme rebound weight / fat gain due to a metabolic reaction to underfueled exercise which is also a factor in men but more extreme in women. Increasing ghrelin and decreasing leptin levels from inadequate fueling of endurance exercise (especially carbs) significantly impacts resting metabolism rates, fertility and many other areas.

        Heidi Mills wrote an article about this topic that was published in Outside magazine April edition in print now – page 70.

        Good luck to everyone!

        1. infinity9

          I don't understand one thing. Here's my story first and then I'll tell you why I'm confused.

          Basically, I'm familiar with what you describe about losing the upper speed gears and inability to perform well at all the higher intensities, including high aerobic steady states, the lactate threshold and the full anaerobic speeds. But I never run into this by decreasing carbs. I don't mess with my diet at all, I sometimes actually force myself a bit to replace the calories.

          I just get this effect from trying to train at lower intensities, slower paces in an effort to increase mileage. I just need 1-2 weeks of such training and my body already goes deep into this "one-gear" mode. How's that possible?! And yes I confirm that it means loss of performance in racing.

          Understandably, I gave up on that way of increasing mileage and since then I've just been doing speed training with easy runs on the other days and I generally do not have this issue anymore. Yet, performance increased, endurance increased, everything. So this verifies what you're saying here.

          Now, however, I've managed to run into the issue again. Recently I started training for a longer distance event (ironman relay, my friend and me, just the two of us). There's unfortunately been limited time to train for it, just 5 weeks. I am less than 2 weeks from the race now. I calculated that in the race I would have to keep moving at a decent pace for about 7 hours so I had to introduce longer workouts than I ever did before.

          In the last big workout I did combo of biking and running for over 4.5 hours. That concludes my training and I'm going to taper now. But, even before that, I was already experiencing the symptoms of losing my speed gears. I wanted to keep a fast paced workout (once a week) in the schedule but I couldn't fit it so my body responded really quickly to all the slower paced workouts by forgetting how to go fast.

          And not only that!! Now I find that symptom of gaining fat is in the picture too!! I didn't have that issue before but in these last couple of weeks I've gained some on my stomach. Lol so much for training at lower intensities(slower paces) and losing weight eh? I am very sure I didn't overeat. If anything, I had trouble replacing all calories especially after the 4.5+ hour workout. I was forcing myself to eat enough. Anyway this just further confirms what you are saying about this "syndrome". Oh and yes I'm female!

          I would ask a question about this experience; even if one doesn't cut back on carbs in diet, the syndrome can come on??? Just by doing too many endurance workouts where possibly part of it is being done on low glycogen??? Though I've never outright bonked.

          However, I will add one more thing I've observed that will just confound things more IMO. I would love to hear what you think about that. So, basically, a while ago, I also experimented with training that was all kept at very low intensities. Nothing else added, it was all even easier paced than normal easy pace. High mileage/lots of time on feet. The lovely thing was I lost weight including fat so quickly without trying to change my diet. I wasn't even trying to lose weight! And the other lovely thing was that my legs didn't necessarily get into this crappy "one-gear" mode. They could stay fresh enough, able to give power, just relatively untrained stamina for faster race paces. I did eventually give up on this way of training too but simply because it didn't really have me improve fast enough for my liking. I now use this very low intensity for my easy paced days that are mixed in the speedwork.

          Can you explain why this even lower intensity didn't cause the issues?

          Explanation by anyone would be nice.

  25. Gary Gellin

    Dr. Cooper – thank you for providing this information. It is almost a radical view these days to speak highly of carbohydrates.

  26. Minh-Hai Tran, MS, R

    As a board certified sports dietitian, I am seeing the same thing in my practice that Dr. Cooper sees: athletes under-fueling, particularly undereating carbs… Maybe they feel great at first, but then wonder why they keep struggling with fatigue and rebound weight gain (even though they're training so hard).

    Just as you train your heart and lungs with endurance exercise, you can also train your gastrointestinal system to tolerate more fuel…a much more proven way to make performance gains!

    Btw, any females reading this…contrary to popular belief, it is NOT normal to miss periods due to training!! (Unless you're menopausal.) Please consult with a sports dietitian, preferably board certified (CSSD) to fine-tune your sports nutrition.

  27. Lori

    A big thank you to Minh-Hai and Dr. Cooper for what you posted. I get really frustrated when I hear about distance runners purposefully depleting their system of what it needs to keep going. This does not make any sense at all, from a logical or scientific stand point. What it will do, in fact, will ensure that the runners who follow this advice will not be able to continue running or reaching their goals or dreams. I can speak from experience. After being sidelined for 14 months as a result of a metabolic condition, once the damage is done, good luck making a fast recovery. Why would anyone test fate and do this to themselves purposefully? It is a form of an eating disorder to restrict the calories necessary for the tasks you are putting your body through. For anyone who has experienced the bonk, while working out, you do that too many times and it will start to happen even when you aren't working out. Have fun laying on the couch. That is all I have to say…and check with at least 3 qualified physicians before you purposefully deplete your body of what it needs while simultaneously asking it to take you to the limits.

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